Properly color taskbar nav buttons when slide-in view is shown.

Supports overriding the current nav bar color with one that would be
visible over a slide-in view.

Demo:
https://drive.google.com/file/d/1MB26QM5qeeyVB6Bj14IGoPCWVT_P7K-3/view?usp=sharing&resourcekey=0-wnMgciNrUXC_l7gI3JFm8A

Test: Manually with light and dark themes, as well as All Apps and EDU
views.
Fix: 248359789

Change-Id: I134ae719b10f3de78ea18220a835de56a82ecb02